Photographic processes



United States Patent 3,155,506 PHOTOGRAPHIC PRUCEdSES Ralph Kingsley Blake, Westfield, Ni, assignor to E. L du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Filed Feb. 18, 1964, Ser. No. 345,589 13 Claims. (Cl. 96-50) This invention relates to photography and more particularly to new processes for forming negative images.

This invention is a continuation-inpart of my prior application Ser. No. 158,131, filed December 8, 1961.

The principal processes of photography are based on the use of colloid-silver halide emulsion layers. In the prior art processes a latent image is formed by imagewise exposure of a radiation-sensitive silver halide emulsion layer. Silver halide bearing a latent image has been developed to silver by selective reduction in these instances.

An object of this invention is to provide a new, simple and practical process for producing photographic images. Another object is to provide such a process which is simple and utilizes commercially practical and economical silver halide layers. A further object is to provide such a process Which utilizes conventional exposure apparatus and reducing and fixing compositions and baths. Still other objects will be apparent from the following description of the invention.

In my prior application Ser. No. 236,420, tiled November'8, 1962, entitled Photographic Processes, and in the prior applications referred to therein and constituting a part of the disclosure thereof, there are disclosed and claimed photographic compositions and layers and positive image-forming processes. The compositions thereof comprise silver halide crystals having intimately associated therewith various compounds including a benzoxazole compound containing at least one --Xl-i group where X is sulfur or selenium, preferably the former, the silver salt of said compound being insoluble in aqueous ammonium hydroxide at pH 12, the solubility of the associated silver halide crystals being aficcted by exposure to actinic radiation, e.g., ultraviolet, visible or infrared light, to such an extent that at least 20% of the less soluble crystals remain after 90% of the more soluble crystals dissolve when the element is treated with aqueous sodium thiosulfate. In general, the organic compound surrounds the silver halide crystals and may be I present as a layer of molecular or greater thickness about the crystal and is present in substantially greater amounts than fog-inhibiting amounts. The organic compounds containing the XH group apparently form a salt, i.e., a mercaptide with the silver halide. Upon exposure to intense actinic radiation, the surface of the treated silver halide crystal, i.e., a silver mercaptide skin, is modified so that the silver halide dissolves markedly more rapidly than unexposed silver halide in a silver halide solvent, e.g., aqueous sodium thiosulfate.

Preferred compositions or layers that can be used in accordance with the processes of this invention are dispersions or emulsions of light-sensitive silver halide grains in a water-permeable organic colloid and they are prepared by admixing With the silver halide emulsions, after precipitation, an organic nitrogen-containing, silver saltforming compound as defined below. The compound can be added to the aqueous colloid silver halide emulsion prior to coating onto a suitable support or by bathing or impregnating the emulsion layer with a solution, e.g., an aqueous solution of the organic compound containing the SH group.

The process of the present invention comprises the sequential steps, in the following order, of

(a) Exposing to intense actinic radiation, by means of a light pattern, a light-sensitive silver halide emulsion layer having a predetermined pH at which, prior to exposure, the silver halide crystals having associated therewith in substantially greater than fog-inhibiting amounts an organic nitrogen-containing silver-salt-forming compound having in one of its tautomeric forms a s H C group attached to an NH- group and to at least one atom selected from the group consisting of sulfur, oxygen and nitrogen atoms, dissolve fairly rapidly in a silver halide solvent; and

(b) Treating the entire surface of the exposed layer with a liquid solution of a silver solvent to remove soluble silver halide in the unexposed areas leaving a silver salt image in the exposed areas; the concentration of the specific nitrogen-containing compound and the predetermined pH being such that when said emulsion layer is exposed to actinic radiation, the solubility of the associated silver halide crystals is affected to such an extent that at least 20% of the less soluble, exposed, crystals remain after of the more soluble, unexposed, crystals dissolve when the layer is treated With 10% by weight aqueous sodium thiosulfate.

When a suitable compound is admixed at the predetermined pH with an aqueous dispersion of silver chlorobromide, 70/30 mole percent, in gelatin containing 57 g. of gelatin per mole of silver and 0.57 mg. of silver per ml. of dispersion and said dispersion is placed in a clear heat-resistant glass test tube 12 mm. in diameter and (1) Exposed at a distance of 6 inches for a period up to 3 minutes, e.g., 3 min., to a 500-watt incandescent tungsten filament lamp having a maximum beam candle power of 50,000 at 10 feet with a mean color temperature of 3400" K., and

(2) Treated with 10% by weight aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver and mg. of sodium thiosulfate), at least three times the amount of silver chlorobromide remains undissolved as in a similar dispersion which was treated similarly except for omission of any exposure to actinic radiation.

The silver halide image-bearing element that results from step (b) is a useful element for various purposes, including images for viewing by projection. This is because the silver salt image is clearly visible.

According to another aspect of the invention Where it is desired to intensify the image resulting from steps (a) and (b), the element is (c) Treated in an aqueous solution containing at least one silver halidereducing agent.

Between steps (b) and (c) the element can be washed with water. Also, after step (c) the element can be washed with water and dried.

The imagewise solution of the exposed silver halide stratum may be affected by the commonly used photographic fixing agents, e.g., sodium thiosulfate, sodium thiocyanate, concentrated solutions of potassium bromide, etc.

Reduction of residual silver halide may be accomplished by use of any chemical reducing agent capable of reducing silver ion to silver metal, e.g., hydroquinone, metol, sodium hydrosulfite and stannous chloride. The function of the reducing agent may be enhanced in the presence of alcohol, thiourea, potassium iodide, etc. The silver image may be toned, e.g., with sodium sulfide, sodium selenide, etc. In addition, color images may be obtained by developing the residual silver halide in the presence of a color coupling compound either in the developing bath or previously incorporated in the emulsion in the manners known to the art.

The pH of the silver halide emulsion can be adjusted by addition of conventional compounds used for adjusting pH in silver halide emulsions, e.g., sodium carbonate, hydrochloric acid, sodium hydroxide, acetic acid, etc.

The silver halide in the silver halide emulsion layers need not be a combination of silver chloride and silver bromide. but may be silver chloride, silver bromide and other mixed halide systems conventional in photographic practice, e.g., silver bromoiodide. While, for rapid processing, a high silver halide to hinder ratio is preferred as described in the examples, more conventional ratios can also be used.

In place of part of the gelatin, other natural or synthetic water-permeable organic colloid binding agents can be used and in some cases such binders can be used alone. Such agents include water-permeable or water-soluble polyvinyl alcohol and its derivatives, e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers and acetals containing a large number of CH CHOH groups, hydrolyzed interpolymers of vinyl acetate and unsaturated addition polymerizable compounds such as maleic anhydride, acrylic and methacrylic acid esters and styrene. Suitable such colloids of the last mentioned type are disclosed in US. Patents 2,276,322, 2,276,323 and 2,397,- 866. The useful polyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents which can be used include the poly-N-vinyllactams of Bolton US. Patent 2,495,918, various polysaccharides, e.g., dextran, dextrin, etc., the hydrophilic copolymers in Shacklett US. Patent 2,833,650, hydrophilic cellulose ethers and esters, and polymers of acrylic and methacrylic esters and amides.

The emulsions may optionally contain any of the usual adjuvants customarily employed in silver halide systems so long as they do not interfere with the adsorption and complexing action of the essential ingredient of the invention.

The emulsions can be coated on any suitable support, e.g., cellulose esters, cellulose mixed esters; superpolymers, e.g., polyvinyl chloride co vinyl acetate, polyvinyl acetals, butyrals; polystyrene; polyamides, e.g., polyhexamethylene adipamide; polyesters, e.g., polycarbonates, polyethylene terephthalate, polyethylene terephthalate/ isophthalate, esters formed by condensing terephthalic acid and its derivatives, e.g., dimethyl terephthalates with propylene glycol, diethylene glycol, tetramethylene glycol, cyclohexane-1,4-dimethanol (hexahydro-p-xylene dialcohol); paper, metal, glass, etc.

This invention will be further illustrated by but is not to be limited to the following examples:

EXAMPLE I A photographic element was prepared by coating an aqueous gelatin dispersion of silver chlorobromide (70 mol percent silver chloride and 30 mol percent silver bromide) on a film base prepared as described in Example IV of Alles U.S. Patent 2,779,684. The dispersion had a ratio of silver halide to gelatin of 28:1 by weight and was coated at a pH of 6 at a rate of 116 milligrams of silver halide per square decimeter. After dry ing, the element was bathed for about 30 seconds in a dilute aqueous solution of Z-mercaptobenzoxazole having a pH of 5.1 and was dried. The latter solution was prepared by diluting 5 ml. of a stock solution (1 gram of the compound made up to 100 ml. in ethanol) with an additional 20 ml. of ethanol and 10 ml. of water. By weighing, it was determined that about 2.2 g. of the compound per mole of silver halide was added by the bathing treatment.

The dried element was then exposed behind a photographic transparency for one minute to the radiation from a General Electric 2A photoflood lamp at a distance of about 6-10 inches. The exposed element was then immersed in 12.8% aqueous sodium thiosulfate for 30 seconds. Subsequently, the fixed film was rinsed briefly in water and then bathed in a rapid photographic developer solution containing 1-phenyl-4-methyl-3-pyrazolidone and hydroquinone as reducing agents and 2.5 g. of KI per liter, to reveal a negative image of the original transparency. All of the above operations were carried out in ordinary fluorescent room illumination.

EXAMPLE II A lithographic emulsion having a silver halide composition of 30 mole percent AgBr and 70 mole percent AgCl and having 20 grams of gelatin present per mole of silver halide for the steps of precipitation and ripening was freed of unwanted, soluble, by-product salts by a coagulation and wash procedure as taught in Waller et al., US. 2,489,341, wherein the silver halide and most of the gelatin were coagulated by an ionic wetting agent, sodium lauryl sulfate, using an acid coagulation environment. It was assumed that the gelatin content decreased from 20 to 10 grams per mole during washing. Following the washing step, the emulsion coagulate was redispersed in water together with 47 grams of additional bulking gelatin (making a total gelatin content of 57 grams per mole). The dispersion was then diluted with water to the extent that 1 ml. of dispersion contained 1 mg. of silver halide (calculated as silver bromide) or 0.57 mg. of silver.

Under safelight conditions (Wratten l-A filter or equivalent), 10 ml. samples of the above dispersion, at a concentration such that 10 ml. of dispersion contained 10 mg. of silver halide calculated as silver bromide, were placed in 5 small beakers. To each of these samples was added a small volume of a solution (1% by weight of solute to volume of solution) of an organic compound in the solvent indicated in Table A which follows. After suitable agitation of the solutions in each of the 5 beakers, two 0.5 ml. samples were transferred to 12 x mm. Pyrex test tubes, one of which was kept as a control under safelight conditions; the other was exposed to a SOD-watt (No. 2) photospot lamp (a tungsten filament incandescent electrical lamp having a maximum beam candlepower of 50,000 at 10 ft. with a mean color temperature of 3400 K.) at a distance of 6 inches for 1 minute. Two-tenths of a ml. of '10% aqueous sodium thiosulfate was then added to each of the ten test tubes, which were briefly agitated and compared under safelight conditions. In all cases, the exposed samples had greater turbidity than the unexposed samples. Thus, the exposure to light caused the silver halide crystals, treated with the organic compound, to be less soluble in aqueous sodium thiosulfate than similar but unexposed crystals. This photoinsolubilization effect was quite dependent upon pH. The values of pH listed in the table below, while not necessarily optimum, do represent a pH at which the particular organic compounds exhibit the effect of photoinsolubilization.

Table A 6 Vol of solution Organic Compound Structure Solvent m g i i lir 1 12.]- p132 ide (cald as AgBr), m1.

S N-methylthiourea r-NH- NHz Hi 0.1 8.0

S N-phenylthiourea @NH- NH1 EtOH 0.1 s. a

S 1 ,5-diphenyl-Zi-thiocarbohydrazid NHN E -NHNH@ Acetone.-. 0. 1 5. 2

Rhodanine 0=C-NH EtOH. 0.5 4. 4

z-thiobarbituric acid 0=ONH EtOH.. 0.1 5. 5

Hz!) (IJ=S o= J'1 in EXAMPLE III A silver chlorobromide film was coated as described in Example 1 except that the coating weight, calculated as silver bromide, was 106 mg. per square decimeter. After drying, a sample of the element was bathed for about 30 seconds in a solution of parts ethanol-+10 parts Water containing 50 mg. of phenyl thiourea and having a pH of 12.8 and was dried. A second sample of the element was bathed for a similar period of time in a similar solution (same solvent and solute cc.) of phenyl biguanide mercaptobenzoxazole at a pH of 10.4 while a third sample was treated similarly with a solution of 1- phenyl-S-mercaptotetrazole at a pH of 1.4.

The dried samples were all exposed as described in Example I, the distance from the lamp being 6 inches and the exposure time 80 seconds. The samples were then immersed in 12.8% aqueous sodium thiosulfate for seconds and in all cases there was obtained a negative silver halide image. Such an image is useful without further treatment, particularly for projection via specular light which elfects an increase in contrast. In the present instance, the samples were further processed by rinsing briefly in water and bathed for 30 seconds in the rapid photographic developer solution described in Example I. This last treatment intensified the image by reducing the silver halide to metallic silver. Good images were obtained with the first two samples but the third sample (treated with the mercaptotetrazole) had a very weak image. All of the above operations were carried out in ordinary fluorescent room illumination.

Where a more sensitive product is prepared by appropriate selection of such factors as grain size, silver halide composition, etc., it may be desirable to use conventional photographic darkroom handling of the element prior to image exposure. During the developing step, the treated element is converted into an exact reproduction (i.e., negative image) of the original transparency. After briefly washing in water and drying, it is satisfactory for use in any application where an exact reproduction is desired, e.g., in the graphic arts field, for a projection transparency, etc.

The photographic processes of the invention likewise have advantages over previously known systems based on selective reduction of exposed silver halide for forming either direct positive or negative images without resorting to the special effects and sensitizing procedures previously used for preparing such images. In addition, since image formation does not require selective reduction; this present process is not limited to the use of certain photographic developing agents but may be accomplished by a wide range of reducing agents. Many such compounds are of very low cost and can be used to form images of much higher covering power than customary, thus elfecting important economies in processing, as well as greatly increasing the efficiency of the silver image with a resultant increase in sensitivity. Still additional advantages will be apparent from the above description of the invention.

I claim: 1. A process which comprises the sequential steps, in the following order, of

(a) exposing to actinic radiation, by means of a light pattern, a light-sensitive silver halide emulsion layer having a predetermined pH and comprising silver halide crystals having associated therewith in substantially greater than fog-inhibiting amounts an organic nitrogen-containing, silver-salt-fonning compound having in one of its tautorneric forms a S H .g..

group attached to an NH-- group and to at least one atom selected from the group consisting of sulfur, oxygen and nitrogen atoms; and (b) treating the entire surface of the exposed layer with a liquid solution of a silver halide solvent to remove soluble silver halide in the unexposed areas leaving a silver salt image in the exposed areas;

the concentration of the specific nitrogen-containing compound and the predetermined pH being such that when said emulsion layer is exposed to actinic radiation, the solubility of the associated silver halide crystals is affected to such an extent that at least 20% of the less soluble, exposed, crystals remain after of the more soluble, unexposed, crystals dissolve when the layer is treated with 10% by weight aqueous sodium thiosulfate.

2. A process according to claim 1 wherein said emulsion layer is a gelatin emulsion layer.

3. A process according to claim 1 wherein said silver halide emulsion layer is a gelatino-silver chlorobromide emulsion layer.

4. A process according to claim 1 wherein said silver halide emulsion layer is a gelatino silver chlorobromide emulsion layer and said silver-salt-forming compound is Z-mercapto-benzoxazole.

5. A process according to claim 1 wherein said silver halide emulsion layer is a gelatino-silver chlorobrornide 7 emulsion layer and said silver-salt-forming compound is N-methylthiourea.

6. A process according to claim 1 wherein said silver halide emulsion layer is a gelatino-silver chlorobromide emulsion layer and said silver-salt-forming compound is N-phenylthiourea. I V

7. A process which comprises the sequential steps, in the following order, of

(a) exposing to actinic radiation, by means of a light pattern, a light-sensitive silver halide emulsion layer having a predetermined pH and comprising silver halide crystals having intimately associated therewith in substantially greater than fog-inhibiting amounts an organic nitrogen-conta'ming, silver-salt-forming compound having in one of its tautomeric forms a group attached to an NH- group to at least one atom selected from the group consisting of sulfur, oxygen and nitrogen atoms,

(12) treating the entire surface of the exposed layer with a liquid solution of a silver halide solvent to remove soluble silver halide in the unexposed areas leaving a silver salt image in the exposed areas;

the concentration of the specific nitrogen-containing compound and the predetermined pH being such that when said emulsion layer is exposed to actinic radiation, the solubility of the associated silver halide crystals is aifected to such an extent that at least 20% of the less soluble, exposed crystals remain after 90% of the more soluble, unexposed crystals dissolve when the layer is treated with 10% by weight aqueous sodium thiosulfate, and

(0) developing the layer in an aqueous solution containing at least one silver reducing agent.

8. A process according to claim 7 wherein said emulsion layer is a gelatin emulsion layer.

9. A process according to claim 7 wherein said silver halide emulsion layer is a gelatino-silver chlorobromide emulsion layer.

10 A process according to claim 7 wherein said silver halide emulsion layer is a gelatino-silver chlorobromide emulsion layer and said silVer-salt-forming compound is Z-mercaptobenzoxazole.

11. A process according to claim 7 wherein said silver halide emulsion layer is a gelatino-silver-chlorobromide emulsion layer and said silver-salt-forming compound is N-methylthiourea.

12. A process according to claim 7 wherein said silver halide emulsion layer is a gelatino-silver chlorobromide emulsion layer and said silver-salt-forming compound is N-phenylt'niourea.

13. A process according to claim 7 wherein the film is Washed in water after steps (b) and (0).

References Cited by the Examiner Mees: The Theory of the Photographic Process; Macmillan, 1942, pages 306309.

Faerman et al.: The Photographic Action of Z-Mcrcapto Benzoxazide, Uspekhi Nauchnoi Fotografii, Akaemiya Nauk, S.S.S.R., Otdelenie Khimicheskikh Nauk 5, 107-113 (1957).

Van Veelen et al.: Phot. Korr. 99, No. 9, September 1963. Pages 139-445. 

1. A PROCESS WHICH COMRPISES THE SEQUENTIAL STEPS, IN THE FOLLOWING ORDER, OF (A) EXPOSING TO ACTINIC RADIATION, BY MEANS OF A LIGHT PATTERN, A LIGHT-SENSITIVE SILVER HALIDE EMULSION LAYER HAVING A PREDETERMINED PH AND COMPRISING SILVER HALIDE CRYSTALS HAVING ASSOCIATED THEREWITH IN SUBSTANTIALLY GREATER THAN FOG-INHIBITING AMOUNTS AN ORGANIC NITROGEN-CONTAINING, SILVER-SALT-FORMING COMPOUND HAVING IN ONE OF ITS TAUTOMERIC FORMS A 